Method of comminuting materials preferably conducting materials, and an apparatus for accomplishing the same

ABSTRACT

A method for the comminution of materials under the effect of current wherein the material to be comminuted is first shaped in the form of a stream or jet and thereafter fed in the interelectrode space. An apparatus for accomplishing said method which comprises a working chamber which accommodates at least one hollow and one earthed electrode, the material to be comminuted being fed as a stream or jet shaped by the hollow electrode into the space between said electrodes.

United States Patent 1 Kassir et a1.

[ METHOD OF COMMINUTING MATERIALS PREFERABLY CONDUCTING MATERIALS, AND AN APPARATUS FOR ACCOMPLISI'IING THE SAME Kemerovskoi oblasti, all of U.S.S.R. I

221 Filed: Apr. 8, 1971 211 Appl. No.: 132,426

said electrodes.

[ 51 Nov. 6, 1973 Primary Examiner-Granville Y. Custer, Jr. Attorney-Holman & Stern [57] ABSTRACT A methodfor the comminution of materials under the effect of current wherein the material to be comminuted is first shaped in the form of a stream or jet and thereafter fed in the interelectrode space.

An apparatus'for accomplishing said method which comprises a working chamber which accommodates at least one hollow and one earthed electrode, the material to be comminuted being fed as a stream or jet shaped by the hollow electrode into the space between 7 Claims, 2 Drawing Figures Ni v 5 i5- Si 7 ///////Ai i 8 '1 7 2 1 1 V I E l=: 2 l S 2 17F i 5' i Q.- 7

i p Lgklfl PATENIEU NUV 6 I973 AN APPARATUS FOR ACCOMPLISHI NG THE SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionirelates to a method of and an apparatus .for comminuting materials, preferably conducting materials, and may find application in the mining industry and powder metallurgy as well as for the manufacture of abrasives, paints and varnishes, and the like.

2. Description of Prior Art A method is known in the art for the comminution of conducting materials which comprises passing a pulsating current between two electrodes immersed in an insulating liquid/that contains the particles of a material to be comminuted, wherein the material being comminuted acts as a conducting medium and in the insulating liquid there occurs spark breakthrough.

An apparatus for embodying this prior art method comprises a working chamber which houses electrodes and contains an insulating liquid with the grains of a material to be comminuted suspended therein.

Said method for size reduction of metals and other conducting materials relates to the field of electroerosion and consists essentially is subjecting the microvolumes of macroparticles (grains) to local heating at the sites of electric spark incorporation into the particles being comminuted, followed by detachment of molten material droplets and subsequent solidification of said droplets in the insulating medium.

In the method described hereinabove, the energy of current pulses is expended not only in a useful manner to obtain finely divided materials, but is also lost due to spark breakthrough and liquid heating in the interparticle space, the losses associated with these phenomena being comparable with the pulse energy. Hence, the practical accomplishment of said prior art method involves enhanced power requirements per ton of finished pulverulent material.

It will also be noted that said known method results in material comminution to a particle size of from 0.05 to 8pm, the particles of the pulverulent material thus obtained being spherical in shape, said method does not make it possible to effect size reduction so as to produce larger particles or other than spherical particles, e.g., fragments.

The application of said method for comminution of conducting materials, such as magnetite, may lead to objectionable undersize.

SUMMARY OF THE INVENTION The primary object of the present invention is to provide a method of and an apparatus for the comminution of materials, preferably conducting materials, which will make it possible to minimize specific power'requirements.

A further object of the present invention is to provide a method of and an apparatus for the comminution of materials which will make it possible to obtain comminuted materials of the desired particle size and shape.

A still further object of the present invention is the provision of an apparatus which will be simple in design and reliable in operation and will practically eliminate objectionable undersize.

I With these and other objects in view, in a method for comminuting materials, preferably conducting materials, by the effect of current pulses on the material being comminuted provision is made, according to the present invention, for imparting to the material to be comminuted, prior to the effect of current pulses thereon, the shape of a stream and directing said stream to the electrode zone where the material undergoes size reduction. I

It is expedient to melt solid materials prior to material stream formation or else to resort to a pulp stream, the pulp consisting of the particles of a material to be comminuted in an insulating liquid.

- It is further advantageous, for the most part, to carry out material size reduction in a compressible medium.

Current pulses may have a duration of 5 to 1,000 microseconds.

The duration of current pulses lends itself to variation by changing the inductance of a discharge circuit.

In an apparatus for embodying the'present method of material size reduction comprising a working chamber with electrodes disposed therein, provision is made, according to the invention, for at least one hollow electrode intended for shaping the material to be comminuted in the form of a stream and feeding said stream into the interelectrode space between said hollow electrode and an earthed electrode.

The hollow electrode may be mounted in the working chamber so as to be capable of being displaced longitudinally.

It is further preferable that in the working chamber provision is made for mounting around each hollow electrode a rigid screen which confines the interelectrode space.

For a better understanding of the spirit of the present invention, pertinent general concepts are discussed hereinbelow.

It is known that the pulse current, which effects material size reduction, flows in a discharge circuit comprising a pulse generator, conductors, and an interelectrode gap, each element of the discharge circuit displaying an internal resistance denoted as R,, R and R wherein R stands for the pulse generator resistance, R; is the resistance of the conductors, and R is the resistance offered by the interelectrode gap. The discharge circuit efficiency, 17, which defines the amount of the energy (and, hence, specific power requirements) evolved in the gap between the electrodes maintained at differnet potentials, canvbe found from the equation It follows from this relationship that the efficiency of a discharge circuit, provided R and R are constant, is directly proportional to the resistance of the interelectrode gap which resistance, in turn, is governed by pulse parameters and the conditions of discharge path shaping.

The resistance offered by the interelectrode (working) gap is negligible in case discharge path shaping occurs between two electrodes immersed in an insulating liquid that contains conducting material particles. With analogous pulse parameters, the discharge path, in case the material to be comminuted travels in the form of a slurry stream in an insulating liquid, provides for an interelectrode gap resistance which will be by an order of magnitude greater than the resistance inherent in the system discussed earlier. This situation owes its origin to the fact that the slurry stream has a substantially smaller cross-section and a significantly smaller number of electrode-'to-electrode current paths.

Hence, feeding the material to be comminuted streamwise to the interelectrode zone where size reduction occurs is advantageous inasmuch as it results in minimizing specific power consumption.

When the material to be comminuted is molten and fed as a jet into the present apparatus, the jet is characterized, because of random motion of molecules at high temperatures, by high resistance, like that of a pulp stream and, therefore, will involve low specific power requirements. It is appropriate to note in this connection that the employment of melt jets for effecting the size reduction of materials, particularly metals or alloys, enhances markedly the beneficial effect referred to above.

As pointed out earlier, the prior art method for material size reduction involves, in the ultimate analysis, melting the entire volume of the material contained in the interelectrode space, the invariably spherical shape of the comminuted material particles indicating that this is indeed the case, and also involves energy consumption for breaking away the molten droplets from macroparticles of the feed material. It follows from the foregoing that in the prior art method a part of the electrical energy is expended on raising the temperature of solid particles to the fusion point.

Metals and alloys are produced, for the most part, by conventional metallurgical processes which call for feed stock melting in diverse furnaces, so that the method of the present invention can be used to advantage for dispering metal or alloy melts obtained in metallurgical furnaces. In this instance, energy is expended for disrupting the continuity of a melt stream fed into the apparatus, the consumption of energy for heating the material to its fusion point being avoided.

A notable distinctive feature of the present invention consists in that the process of material size reduction is effected in a compressible medium, e.g., in a gas. Current pulse flow through a jet of the molten material being comminuted results in the occurrence of such phenomena as the disruption of jet continuity, shock wave onset, and the resultant spread of the droplets thus formed at a velocity of greater than 100 m/sec. Upon impact against a rigid barrier provided by the working chamber walls or by special screens that confine the interelectrode space, the droplets undergo additional destruction.

It is practicable to vary the comminuted material particle size by varying the parameters of current pulses, as well as by altering the cross-section or linear dimensions of the melt jet. Changing the conditions of cooling the spreading droplets and also the distance from the melt jet to the screen makes it possible to obtain the comminuted material particles in the form of spheres or fragments depending on whether the spreading material collides with the screen in the liquid or the solid state.

Where a current pulse is' sent via a slurry stream, there occur breakthrough phenomena in the interstices between the particles of the material being comminuted, said interstices being filled with the insulating liquid, and the electrodes will be closed by the resulting discharge path which serves as a pulse current conductor. The discharge path passes through a part of the particles of the material being comminuted and also through the insulating liquid that fills the interstices between said particles. The material particles which conduct the pulse current are heated to a temperature at which there occurs thermal explosion and partial sublimation of said particles.

The products of thermal explosion consist of finely divided spherical particles of the comminuted material. In the insulating liquid that surrounds the discharge path, there forms a rapidly expanding vapour-gas cavity which sets on a shock wave, said shock wave being instrumental in comminuting a part of the material that adjoins the discharge path and producing irregularly shaped particles. The particles that underwent size reduction due to the effects of the thermal explosion and shock wave, as well as intact material particles contained in the pulp stream between the electrodes, fly apart in all directions from the discharge path. The velocity of the particles of the material being comminuted has been found to depend on the rate of energy supply to the discharge channel and may be as high as 200-250 m/sec. The solid material particles flying at this velocity collide with the rigid screen and undergo disintegration with the resultant formation of fragments. The ratio of the particles that were comminuted as a result of thermal explosion and shock wave phenomena to those produced by impact upon the rigid screen was found to be dependent, provided the pulse energy is constant, on current pulse duration. When pulse duration is increased up to a certain limit by raising discharge circuit inductance, the comminuted materials contains as much as percent of round-shaped particles. Conversely, shorter pulses produced by diminishing discharge circuit inductance increase the proportion of particles produced by impact against the rigid screen, so that at a definite duration of the current pulses the resultant comminuted material contains percent of irregularly shaped particles (fragments).

The feasibility of material comminution so as to obtain particles of various size by the effect of pulse current on a slurry stream prepared from the material to be comminuted is illustrated hereinbelow by the description of the operation of the apparatus according to the present invention.

The mechanism of non-conducting material size reduction appears to be as follows. In this instance, the discharge path involves the slurry stream liquid component only, whereby a shock wave is generated within the liquid, the effect of said shock wave manifesting itself in that solid particles in close vicinity to the discharge path undergo size reduction, while particles disposed at a greater distance from the discharge path where shock wave attenuation is marked are not disintegrated, but nonethless acquire a velocity which might be adequate to produce therein breaking stresses once the particles collide with the rigid screen. The fact that here the process of mechanical disintegration of particles is operative is responsible for the irregular shape of comminuted material particles (fragments).

BRIEF DESCRIPTION OF THE DRAWING The present invention will best be understood from the following description of a specific embodiment of the apparatus for carrying out the process of the invention when taken in connection with the accompanying drawings, wherein:

DESCRIPTION OF PREFERRED EMBODIMENTS The apparatus shown in the drawings comprises a working chamber 1 (FIGS. 1. and 2) which is built up by shell 2, provision being made in the shell 2 for a tapering bottom 3 that ends in an outlet connection 5!, and an electrically insulated cover 5, hollow electrodes 6 mounted on said cover 6 being capable of longitudinal traveLThe working chamber 1 houses rigid screens 7 arranged so as to confine the space 8 between each electrode pair consisting of a hollow electrode 6 and an earthed electrode 9.

To mount the rigid screens 7 and the earthed electrodes 9 in the working chamber 1, use is made of horizontal cross pieces 10 and braces 11. The rigid screens and earthed electrodes are made replaceable.

Apparatus operation is illustrated hereinbelow with reference to one electrode pair.

The material to be comminuted is fed to the hollow electrode 6 (FIG. 1), which effects shaping the stream of said material. The stream issuing from the hollow electrode 6 enters the interelectrode space 8 and travels as far as the earthed electrode 9, whereupon a current pulse passes through the stream, the attendant effects being thermal explosion of the material particles and shock wave generation. These phenomena bring about partial comminution of material particles and cause both comminuted and intact material particles to travel at a high velocity, the resultant collision of said particles with the rigid screen 7 leading to a further particle size reduction. The thus comminuted material falls through the ports (not shown in the drawings) in the cross piece 10 (FIG. 10) onto the tapering bottom 3 and leaves the working chamber 1 through the outlet connection 4. 4

Specific embodiments of the present method are described in the following examples.

EXAMPLE 1 To prepare a weighting agent for heavy suspensions used in mineral benefication, an alloy'consisting of 85% Fe and percent silicon is subjected to comminution to obtain spherical particles under 0. 10 mm in dia. The alloy is melted in an electric furnace and the melt is poured into the apparatus through the hollow electrode 6 (FIG. 2) 2 mm in dia. Once the melt stream reaches the earthed electrode 9, a current pulse of about 50 MW is applied to said stream, whereby the stream is dispersed to yield essentially spherical particles under 0.1 mm in dia., the particles thus produced being directed via the outlet connection 4 to a receiving bin or to screening. Comminuted material oxidation is prevented by filling the working chamber 1 with an inert gas, e.g., argon.

In the preferred operating mode, the time required for closing the interelectrode space with the material stream should be equal to the pulse ratio.

EXAMPLE 2 The comminuted alloy of Example I with particles in the 0.10 mm to 2.5 mm range, which are discarded as oversize for the application specified in Example 1, is

mixed with an insulating liquid and the resultant slurry pumped to the hollow electrode 6, the electrode diameter being 6 mm. When the slurry stream issuing from the electrode 6 reaches the earthed electrode 9, a current pulse of about 1,200 J is applied to the slurry stream. The thus comminuted material is directed via the outlet connection 4 to a classifier. The standard product (particle size, up to 0.10 mm) is sent to a drying step, while the oversize (particle size, above 0.10

mm) is directed for additional comminution. It is preferable that the operating mode requirements set forth in Example 1 be observed.

When the discharge circuit incorporates an inductance of l to 3 mcI-l, the current pulses provide for a percent content of fragments in the end product, while with an inductance of mold the process yields the product that contains up to 60-70 percent of spherical particles in the size range of up to 63p. m.

Undersize is practically absent, inasmuch as in the apparatus of the present invention the comminuted material will be completely discharged from the comminution zone prior to the passage of the next current pulse.

The insulating liquid should preferably be selected from the group consisting of liquefied gases, such as nitrogen or argon, hydrocarbons, or water.

EXAMPLE 3 Green silicon carbide, which is useful in the manufacture of abrasives, is subjected to comminution by following the procedure of Example 2. The end product is characterized by the particle size specified in Example 2, and the particles are of irregular though essen-. tially isometric shape.

We claim:

1. An electrical method for comminution of materials by using an electrical discharge in a zone between two electrodes of opposite polarity, the method comprising: feeding a stream of the material to be comminuted to said zone in a direction of said electrical discharge, and comminuting said material under the effect of current pulses in said zone between said two electrodes.

2. The method defined in claim 1, which further includes the step of subjecting solid material to be comminuted to melting prior to feeding a stream of the material to said zone. I

3. The method defined in claim 1, wherein the material to be comminuted is fed in an insulating fluid.

4. The method of claim 3, which includes the step of controlling a duration of current pulses between said electrodes to be in the range of 5 to 1,000 microseconds.

5. A method of claim 4 wherein the duration of current pulses is controlled by varying a discharge circuit inductance at a constant pulse energy.

6. An apparatus for the comminution of materials, said apparatus comprising: a working chamber; at least one hollow electrode disposed in said chamber and at least one earthed electrode disposed in said chamber and spaced from said hollow electrode; and an interelectrode space between said electrodes into which the material to be comminuted is fed in the form of a stream through and in the direction of the hollow electrode.

7. The apparatus of claim 6, wherein a rigid screen is disposed in the working chamber around the hollow electrode, said screen encompassing the interelectrode space.

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2. The method defined in claim 1, which further includes the step of subjecting solid material to be comminuted to melting prior to feeding a stream of the material to said zone.
 3. The method defined in claim 1, wherein the material to be comminuted is fed in an insulating fluid.
 4. The method of claim 3, which includes the step of controlling a duration of current pulses between said electrodes to be in the range of 5 to 1,000 microseconds.
 5. A method of claim 4 wherein the duration of current pulses is controlled by varying a discharge circuit inductance at a constant pulse energy.
 6. An apparatus for the comminution of materials, said apparatus comprising: a working chamber; at least one hollow electrode disposed in said chamber and at least one earthed electrode disposed in said chamber and spaced from said hollow electrode; and an interelectrode space between said electrodes into which the material to be comminuted is fed in the form of a stream through and in the direction of the hollow electrode.
 7. The apparatus of claim 6, wherein a rigid screen is disposed in the working chamber around the hollow electrode, said screen encompassing the interelectrode space. 